During the launch of a spacecraft, reaction wheel bearings are subjected to very high vibration loads. These loads are a combination of static acceleration, acoustic, random vibration, sinusoidal vibration, and shock. Additionally, resonant frequencies within the spacecraft and reaction wheel structure may cause significant amplification, resulting in very high loads being exerted on the reaction wheel bearings. While it is possible to use large bearings to react these high vibration loads, large bearings carry a penalty of significant increase in viscous drag and therefore have a much higher power consumption. On spacecraft mass, power, and volume are precious resources, and therefore simply using larger bearings is not an optimal solution.
Spacecraft with 3-axis control typically use 3 or more (for redundancy) reaction wheels, so increasing mass and power consumption of a single reaction wheel has a 3 or 4 times penalty for the spacecraft. All power on a spacecraft is typically generated with solar arrays and batteries, and these are large and heavy, so it is critical to minimize the mass and power consumption of spacecraft components including the reaction wheels.
If small bearings are used to support the reaction wheel assembly, the spacecraft will benefit from minimal power consumption, and the overall system mass will be minimized. However, for a given launch vibration load, small bearings have lower load capacity and therefore will experience high stresses, which can damage the bearings and result in a premature failure of both the reaction wheel and the spacecraft.
It is desirable to minimize loading on the reaction wheel ball bearings. Several different approaches have been applied to solve the problem, but none of the prior approaches have provided a satisfactory solution.
Prior methods have included restraining the heavy rotor of the reaction wheel assembly using a mechanism for axially clamping the rotor to the housing. Other previous methods have been used to capture the rotor web or rim. These offloading and restraint methods all require mechanisms for release, however, and therefore add complexity, mass, and failure modes to the reaction wheel.
Other prior methods have attempted to minimize vibration loading on bearings by tuning the vibration response of the rotor, such that it acts as a tuned-mass-damper. Tuned-mass-dampers require extra mass and volume, and in practice they rely on exact knowledge of the as-built mass properties and vibration response of the rotor and structure. Small errors in the mass properties or stiffness prevent this method from working effectively.
Other prior solutions utilized in the industry have provided off-loading of the rotor mass through the presence of “snubbers”. Snubbers are structural supports that are located very close (i.e. 0.010″ or less) to the rotor. When the rotor is subjected to launch loading, the rotor deflects axially and radially and the small clearance (“snub gap”) becomes zero, thereby transmitting the rotor load to the structure and offloading the rotor load from the bearings. Snubbers are difficult and expensive to successfully use in practice, however, and the very small snub gaps are potential failure modes of the reaction wheel. Small particulates can jam in the small gap, preventing wheel rotation, and causing a catastrophic failure. Furthermore, if small variations in the as-manufactured parts are present, this snub gap could be a different size than expected, potentially creating further failure modes. Too large of a gap may result in ineffective off-loading of the bearings, and too small of a gap could be susceptible to failure. Lastly, it is not unusual for launch vibrations to cause small shifts in bolted interfaces of structures. If this happens, it is possible that a small snub gap could become a source of rubbing, friction, and failure of the reaction wheel. Offloading the reaction wheel through the use of very small snub gaps is therefore expensive to accomplish and risky.
Other prior solutions have integrated fluid viscous damping into the bearing support. While viscous damping may be viable for terrestrial applications, damping fluids are not compatible with the vacuum environment of space, however.
What is needed is a reliable way to isolate vibrations between a body to which a reaction wheel housing is attached and the reaction wheel bearings.